A known example of a riveted joint structure and a riveting method applied to a vehicle body of an automobile, or the like, is disclosed in Japanese Laid-open Patent Publication No. HEI 8-4732.
As shown in FIG. 1, a rivet 150 used in this riveted joint structure is a hollow rivet that consists of a head portion 152 and a cylindrical portion 154 extending from the lower surface of the head portion 152. A distal end portion 154A of the cylindrical portion 154 is inclined with an acute angle formed at its outer periphery, such that the distal end portion 154A is tapered inwards from its outer edge toward its inner edge. With the distal end portion 154A thus formed, the rivet 150 can easily enter or pierce a metal plate or plates with improved efficiency.
In this riveted joint structure, however, when the rivet 150 is inserted into two plates 156 and 158 as a workpiece as shown in FIG. 2, the plate 156 against which the rivet is first pressed is bent and deformed. As a result, a clearance or gap 160 is formed between the head portion 152 of the rivet 150 and the plate 156, thus deteriorating the strength.
Another known example of the riveted joint structure and riveting method applied to a vehicle body of an automobile, or the like, is disclosed in WO 98/31487 published on 23 Jul. 1998.
As shown in FIG. 3A, the rivet disclosed in the above publication is a self-piercing tube-like rivet, which has a cylindrical rivet body 200. As shown in FIG. 3B, axially opposite end portions 200A and 200B have inner circumferential surfaces that are inclined in a direction in which the rivet is driven or inserted into a workpiece, so that the driving load, or the force required to drive the rivet into the workpiece, can be reduced.
In the above-described riveted joint structure, however, the rivet body 200 has a uniform strength over the entire length thereof. In this case, if the strength of the rivet body 200 is increased so as to prevent unnecessary deformation, such as buckling of a middle portion 200C, upon driving of the rivet into the workpiece, the axially opposite end portions 200A, 200B and their vicinities are not sufficiently deformed, resulting in reduced joining or fastening force. On the other hand, if the strength of the rivet body 200 is lowered so as to sufficiently deform the opposite end portions 200A, 200B and their vicinities, thereby to enhance the fastening force, the middle portion 200C suffers from buckling, or the like, upon driving of the rivet into the workpiece. It is thus difficult to achieve both a desired efficiency in driving the rivet into the workipiece, and desired deformation characteristics of the rivet, and is also difficult to provide a sufficiently large fastening or joining force at the same time.
Another known example of a rivet structure used in a vehicle body of an automobile, or the like, is disclosed in Japanese Laid-open Patent Publication No. HEI 2-66707.
In the rivet structure disclosed in the above publication and also shown in FIG. 4, a rivet 300 consists of a head portion 302 and a rod portion 304 extending from the lower surface of the head portion 302. As shown in FIG. 5, the rod portion 304 is formed on its outer circumferential surface with saw-toothed protrusions 306 each having a triangular cross section in a plane perpendicular to the longitudinal direction of the rod portion 304. After the rivet 300 is inserted or set into a workpiece, therefore, the protrusions 306 of the rivet 300 engage with the workpiece, thus preventing the rivet 300 from rotating.
In the rivet structure, however, the protrusions 306 are formed with the same height over the entire length in the longitudinal direction of the rod portion 304, and the distal end 304A of the rod portion 304 has a relatively large area. Thus, a large driving force is required for driving the rivet 300 into a workpiece to be fastened, since the distal end 304 of the rod portion 304 receives large resistance force from the workpiece. Furthermore, the distal end 304a is less likely to deform radially outwardly of the rod portion 304 because of the protrusions 306, and a large driving force is required for this reason as well. Thus, the rivet as shown in FIGS. 4 and 5 is driven into a workpiece with a poor efficiency.